The blood coagulation factors are distributed in plasma, with various types of factors from 1st coagulation factor to 13th coagulation factor working sequentially to result in blood coagulation. The mechanism wherein individual blood coagulation factors participate in blood coagulation is shown in FIG. 1.
As illustrated in FIG. 1, blood coagulation is accomplished through very delicate and complex processes wherein reactions occur sequentially. In general, inactive precursors are activated by certain activated coagulation factors (indicated by “a” at the end of the designation of the coagulation factor), and then the coagulation factors are activated. Most of the activated coagulation factors are enzymes called serine protease. These bind to the surface of platelets to sequentially activate the blood coagulation factors to finally create fibrin clot to complete hemostasis.
Thrombin plays the most important role among them. Thrombin is activated from prothrombin (the precursor) via prothrombinase complex consisting of Va factor, Xa factor, Ca++ and phospholipids (PL). When fibrinogen is converted into fibrin thereby, fibrins are crosslinked by the activated XIIIa factor to finally provide stabilized fibrin clot.
In order to form prothrombinase complex, X factor should be activated to Xa factor; this occurs by means of factor Xase complex. Factors produced via intrinsic pathway such as VIIIa factor, IXa factor, Ca++ and phospholipids (PL), or factors produced via extrinsic pathway such as VIIa factor, tissue factor (TF) and Ca++ serve as the factor Xase complex.
In the meanwhile, thrombin functions to activate V factor and VIII factor. If thrombin is excessively generated, problem of vaso-occlusion may occur. In order to control such a problem, thrombin would run a process for inhibiting blood coagulation. Thus, thrombin binds to thrombomodulin to activate protein C to generate protein Ca, which will bind to protein S to inactivate the activated Va and VIIIa factors.
Factor Xa is one of the proteases associated with complex processes of blood coagulation, catalyzing conversion of prothrombin into thrombin. Thrombin crosslinks with fibrinogen, and then decompose it into fibrin monomer which essentially contributes thrombus formation. Activation of thrombin may induce thromboembolism. However, inhibition of thrombin may inhibit formation of fibrin which is associated with thrombus formation.
Thus, inhibition of factor Xa may prevent formation of thrombin, and the compounds of Chemical Formula (1) according to the present invention and salts thereof inhibit factor Xa, to participate in the processes of blood coagulation, thereby inhibiting thrombus formation.
Among the compounds known as factor Xa inhibitors up to the present, protein inhibitors are antistasin (ATS), tick anticoagulant peptide (TAP), and the like. ATS is a compound consisting of 119 amino acids, obtained from leeches, having Ki value for factor Xa of 0.05 nM, while TAP is a compound consisting of 60 amino acids having Ki value for factor Xa of 0.5 nM. Those protein inhibitors are not employed in clinical practices at present, while heparin, sulfated polysaccharides, or the like is rarely employed with limitation.
Development of inhibitors against blood coagulation, particularly factor Xa inhibitors, as a low molecular compound was disclosed by WO9529189. Factor Xa inhibitors employing indole derivatives are described in WO9933800. Various types of factor Xa inhibitors have been reported, including nitrogen-containing heterocyclic compounds (WO2004058743), imidazole derivatives (WO2004050636), pyrazole derivatives (WO2004056815), indole-2-carboxamide derivatives (WO2003044014), oxybenzamide derivatives (WO2002051831), guanidine and amidine derivatives (WO2002046159), amino-bicyclic pyrazinone and pyridinone derivatives (WO2004002405).
However, those low molecular compounds have to overcome the problems to meet stability in plasma and liver, selectivity from other serine proteases (thrombin, trypsin, cathepsin G, or the like), low toxicity and high bioavailability, as well as high pharmaceutical activity.
The most advanced one among the oxazolidinone type compounds similar to those of the present invention up to the present is Rivaroxaban represented by Chemical Formula (A), of which clinical studies in three aspects are now on progress. Likewise, oxazolidinone derivatives represented by Chemical Formula (B) are disclosed by WO 01/47917. Those compounds, however, have limited solubility yet: for example, about 8 mg/L of solubility for Rivaroxaban. Improvement in this regard is considerably required.

Furthermore, factor Xa inhibitors using Apixaban derivatives represented by Chemical Formula (C) with cyclic amidine group being incorporated or sulfonyl amidine derivatives represented by Chemical Formula (D) were disclosed in WO 2004/83174. However, oxazolidinone derivatives with cyclic amidines (compounds of the present invention) have not been used in this regard.

The keypoint of the studies on FXa inhibitors and thrombin inhibitors in the same cascade having been known up to the present is on the inhibitor comprising P1 group containing benzamidine as an arginine derivative. Since both FXa and thrombin comprise arginine at P1 position, they have similar features of problems. The amidine (including guanidine derivatives) groups which replaces the guanidine moiety of arginine is very hydrophilic, so that most inhibitors containing the groups result in poor or insufficient absorption, and even though they are once absorbed, they frequently exhibit short half-life due to the specific property of high clearance (Drugs of the Future, 1999, 24(7), 771).
Amidine itself has strongly basic structure with the PKa of about 12.5. Considering that insufficient absorbance comes from tendency of being positively charged, replacements with derivatives with low basicity have been tried. Representatives include pyridine derivatives, amidrazone, cyclic amines, alkylamine derivatives, aminobenzisoxazole, and the like (U.S. Pat. No. 6,958,356), and a representative example of investigations to replace the amidine with other derivatives is replacement of amidine with amidoxime. Amidoxim has a structure with hydroxyl group attached to amidine itself, being a prodrug taking advantage that weak N—O bond is easily reduced to amidine in vivo. On the base, it makes use of the fact that PKa of amidoxime (about 8-9) is noticeably lower than that of amidine. Representative example of amidoximes is ximelagatran. Such a tendency is equivalently observed in the studies on FXa inhibitors as well as on thrombin inhibitors. When such various trials face limitations, incorporation of neutral P1 group has been rather tried. However, being different from other drug substances, FXa and thrombin inhibitors are advantageous as the blood concentration increases, and the free drug concentration, without binding with protein in blood, is a very important field. In case of neutral P1 group inhibitor, the pharmacological effect rarely appears because the protein binding is relatively high.
In order to solve the problems described above, the present inventors tried to incorporate a relatively polar group at the other site of the inhibitor, instead of limiting P1 group to neutral groups, in order to 1) improve water solubility and 2) lower the protein binding. Lowered protein binding usually provides beneficial effects in PT assay. Practically, lowered protein binding can be indirectly checked by excellent PT value, in spite of relatively low binding affinity to FXa. Thus, the inventors have proceeded with the studies for increasing pharmacological effect by reducing the protein binding.
The position for incorporation of polar group according to the invention is P4 site. The theoretical background is described below. The S4 site of FXa comprises the binding site of “”-shape with three sides have been surrounded by Tyr99, Phe174 and Trp215. Since the binding site consists only of aromatic amino acid side chains, it is basically different from thrombin surrounded by Leu99, Ile174 and Trp215. During the course of drug design, such differences are positively utilized.
The S4 pocket of FXa has high tendency of interaction with cationic residue, which is usually called “π-cation interaction”. In practice, some inhibitors have been designed and synthesized as a positively charged group at P4 site. The present invention intends to enhance the pharmacological effect by improving water solubility and lowering the protein binding, as described above, via incorporation of cyclic amidines at P4 site. The reason for selecting cyclic amidine rather than non-substituted amidine is that amidine has three NH bonds (substantially four in biological PH) so that it negatively acts in terms of drug-likeness, while a cyclic amidine having 0 to 1 NH group is advantageous from the same aspect. According to recent investigations, it is more advantageous if the number of pharmaceutically acceptable hydrogen bond donor (HBD) decreases less than that of hydrogen bond acceptor (HBA). According to Lipinski's rule, up to ten (10) HBA's are acceptable, but the number of HBD is restricted to five (5) (Adv. Drug Delivery Rev., 2001, 46, 3-26). Average HBD value of known new drugs is about 1.8—being relatively strict on HBD (J. Med. Chem. 2004, 47, 6338-48). From this point of view, it is anticipated that a substituted cyclic amidine (number of HBD=0) is far advantageous as compared to an unsubstituted amidine (number of HBD=3). Though a substituted amidine has noticeably decreased number of NH, the amidine functional group itself still has positive charge, so that separation-purification-storage can be done in salt state, thereby having excellent water solubility.
From the studies of the present invention, these features of compounds of Chemical Formula (1) were practically confirmed. In the experiment section, the water solubility and protein binding values, as well as 2×PT value versus Ki are diagrammed.